Transposable Genetic Elements. We are testing the possibility that transposition of genetic elements too small to be observed by conventional cytogenetic methods may play an important role in development and evolution. We have observed clear-cut differences between restriction digests of embryonic and adult Drosophila melanogaster DNA. These differences are found on gel electrophoretograms of restriction digests, using radioactive cloned Drosophila DNA segments for hybridization according to the method of Southern (1975). Two kinds of change have been observed, that in which a fragment present in a restriction digest of adult DNA is not found in the digest of embryo DNA, and vice versa. We will attempt to determine whether these differences actually result from sequence rearrangement or instead from secondary modification or specific chain discontinuities. Clear-cut differences are also seen between restriction digests of Drosophila melanogaster and its sibling species Drosophila simulans. The possibility that these result from sequence rearrangement will be studied by hybridization in situ. Nuclear RNA. Temperature elevation induces high level transcription at a small number of chromosomal sites in Drosophila. We have isolated polysomal messenger RNA that hybridizes at several of these sites. This is being used to select bacterial plasmids containing heat shock genes and adjacent DNA sequences. Restriction maps will be made of the Drosophila inserts. We will then attempt to determine which restriction fragments hybridize radioactive nuclear RNA extracted from tissue culture cells at various times after temperature elevation. Using this approach, we hope to determine the pattern of transcription of DNA sequences adjacent to structural genes and the possible involvement of promoters relatively distant from structural sequences.